Widely known in the art are piston pumps comprising a cylinder and a piston mounted for reciprocations in the cylinder. The cylinder communicates, via valves, with a source of fluid being pumped and with a delivery line. The piston is driven by a motor by means of a crank gear.
Discharge rate of such a pump may be increased by raising piston speed or increasing the pump size, but any increase in the discharge rate is limited by inertia forces and friction which becomes greater with an increase in the area of friction surfaces. An increase in the number of cylinders with pistons results in a decrease in power-to-weight ratio and efficiency and in an increase in metal weight-to-discharge ratio and also in a more complicated design. All these factors impose additional stringent requirements upon accuracy and finish in the manufacture.
Known in the art is a hydraulic actuator having a casing with an interior space in which a toroidal flexible shell is provided which is mounted for reciprocations and comprises a hose having the ends which are turned inside out and which are individually secured along the perimeter to the inner wall of the casing. An annular partition dividing the interior space into two chambers is provided in the casing between the fixed ends of the hose. The partition has a central opening with a sealing means, the hose passing from one chamber into the other through this opening. The chambers communicate with hydraulic lines for alternately supplying working fluid thereto (cf. SU, 918590).
When the hose passes through the central opening having the sealing means, folds are formed in the hose, and working fluid overflow from one chamber to the other occurs through these folds thus lowering efficiency. In addition, an increased friction occurs in the sealing means which also lowers efficiency and causes substantial wear of the hose and sealing members. The envelope moving almost completely from one chamber into the other requires the apparatus size to be very large.